1. Field of the Invention
The present invention relates to a device which forms an image by means of an electrostatic copying process such as a copier, a facsimile, or a printer.
2. Description of the Related Art
In recent years, with the need for an increase in speed and the energy saving of a copier, a low-melting point toner has been developed. However, if the melting point of the toner is lowered above a certain level, various problems occur such as the development of errors and the fixation of toner onto a developer roller by the aggregation of toner. In particular, when outputting an electronic document, in most cases, a non-image part tends to continue in the same portion (especially, a blank space), and the toner is likely to be fixed onto the developer roller facing the non-image part of the electric document. Such fixation of toner onto the developer roller is likely to develop an undesirable image such as a stained surface.
Because the rotation speed of the developer roller is especially high in a high-speed copier, the temperature of the developer roller is prone to increase, and the low-melting point toner is likely to be fixed onto the developer roller. Moreover, in the non-image part, because an electrostatic force acts such that the toner is pressed onto the developer roller, the fixation of toner tends to occur in the developer roller.
In a negative/positive process (hereinafter referred to as a N/P process) which transfers toners onto an exposed portion of an image carrier, since the toner is charged to a polarity which is the same as a polarity of development bias potential, in an area where the toner is fixed, effective development potential onto an image carrier increases. Therefore, when ensuring a sufficient development performance in an area to which the toner does not adhere, the development performance becomes too great in an area where the toner is fixed, causing an abnormal image such as a stained surface.
In this case, a cooling effect is improved by applying a conductor in a case of a development station. In a technique which applies the conductor in the case, in order to prevent the leakage of a development bias voltage, it is arranged to provide continuity across the case and the surface of the developer roller, and to maintain the electric potential of the case and the development bias potential at the same electric potential. With this structure, in the area of the developer roller to which the toner is fixed, an electric potential difference locally occurs relative to the case; thus, the toner is developed on the case from the surface of the developer roller, and then the adhesion/fixation of toner is developed on a part of the case facing the developer roller. In this case, when the toner fixed onto the developer roller is eliminated by maintenance or the like, the toner is attracted to the developer roller from the case because effective electric potential of the part of the case to which the toner adheres/is fixed is higher than the electric potential of the surface of the developer roller. As a result, the toner is even fixed on the surface of the developer roller corresponding to the non-image part which originally does not cause the fixation of toner.
In order to prevent the fixation of toner onto the developer roller, various techniques are proposed. JP2001-312126A proposes a method of preventing the fixation of toner onto a developer roller, which sets a background potential (difference between charged electric potential of image carrier and development bias potential) to be 400V or more at an absolute value, and develops a solid image on the image carrier in the end portion of the developer roller in the axial direction thereof, in an image forming device with a two-component developer using low-melting point toner. However, in this method, the solid image developed on the image carrier is input to a cleaning station of the image carrier or a cleaning station of a downstream transfer station, so that the load on the cleaning station is increased.
In addition, JP2002-278275A proposes a method of electrostatically removing toner on a developer roller by applying a bias voltage to a collection member facing the developer roller. However, in this method, the mechanism becomes complex by the increase in the number of parts, and the costs of the development station are increased.
Moreover, JP2001-242712A proposes, in order to prevent the fixation of toner onto a developer roller resulting from the stress of developer between the developer roller and an image carrier, a developer roller which prevents the fixation of toner without losing development performance by minimizing a half-value width of a development magnetic pole.
Furthermore, JP2002-278183A proposes an image forming device, which controls a defect of toner concentration control resulting from the fixation of toner onto a developer roller. Because the fixed toner is charged in the toner fixation area of the developer roller, effective development potential is increased. Accordingly, when performing normal control, in the toner fixation area, the toner adhesion amount of a patch for measuring the toner adhesion amount on the image carrier is increased, compared to an area where the toner is not fixed. Consequently, if the concentration of toner is controlled according to the toner adhesion amount of the patch for measuring the toner adhesion amount, a low concentration of toner is set in an area where the toner is not fixed, resulting in the decrease in the concentration of an image. In this image forming device, the development bias potential and the electrostatic latent image potential are controlled to be the same, and the toner adhesion amount of the patch for measuring the toner adhesion amount is measured in this state, so that the increase in the effective development bias potential by the fixation of toner is detected and is used for controlling the concentration of toner.